Semiconductive alloy light source having improved optical transmissivity



ENERGY Jan. 31, 1967 s. v. GALGINAITIS 3,302,051

SEMICONDUCTIVE ALLOY LIGHT SOURCE HAVING IMPROVED QPTICAL THANSMISSIVITYFiled Dec. 12, 1963 Fl'gj Inventor-2r 8 r1 VGZ/ ind/'f/s b) Fe g 4 sAttorney.

DISTANCE United States Patent 6 l 3,302,051 SEMICONDUCTIVE ALLOY LIGHTSOURCE HAV- ING IMPROVED OPTICAL TRANSMISSIVITY Simeon V. Galginaitis,Schenectady, N.Y., assignor to General Electric Company, a corporationof New York Filed Dec. 12, 1963, Ser. No. 330,173 Claims. (Cl. 313-108)The present invention relates generally to semiconductive light sourcesand more particularly pertains to means for etficiently extracting theradiation generated within such a light source.

There are a plurality of semiconductive materials, as gallium arsenide,for example, that are known to provide efiicient electroluminescence inresponse to electrical excitation. A particularly eilicient group, ofsemiconductive light sources, is that wherein the source of radiation isa P-N junction formed in the semiconductive material. Such light sourcesare disclosed, for example, in Physical Review Letters, vol. 9, p. 366(1962), by R. N. Hall, G. E. Fenner, J. D. Kingsley, T. J. Soltys and R.O. Carlson, wherein efficient light production at a P-N junction isdisclosed to produce coherent radiation within the semiconductivemate-rial.

While semiconductive junction light sources exhibit eflicientgene-ration of light within the semiconductive material, the moredesirable semiconductive materials for this purpose possess highdielectric constants, high optical refractive indices and greatabsorptivity. The difficulties associated with the former twocharacteristics are substantially overcome through practice of theinvention by W. Engeler, disclosed and claimed in copending application,Serial No. 330,172, filed concurrently herewith. It would be highlydesirable to reduce the a-bsorptivity of the semiconductive material andprovide a further increase in luminous efiiciency.

Accordingly, it is an object of my invention to provide a more efiicientlight source fabricated from semiconductive materials.

Another object of my invention is to provide a P-N junction light sourcehaving reduced absorptivity of the light generated within the junction.

Still another object of my invention is to reduce the absorption ofradiation within a semi-conductive body.

Briefly, in accord with a preferred embodiment of my invention, Iprovide an alloy semiconductive body wherein the concentrations ofconstituents varies along at least one dimension of the body. Theconcentrations of constituents are selected to provide one region in thesemiconductive body having a lower band-gap energy than the remainder ofthe body. A P-N junction is formed in the low band-gap energy region andthe remainder of the body is adapted to transmit radiation originatingat the PN junction. Thus, radiation originating in the vicinity of thejunction is in quanta that are unable to interact with electrons in thehigher band-gap mate-rial and absorptivity of the portion of thesemiconductive body that is adapted to transmit the radiation is greatlyreduced.

The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, however,both as to its organization and method of operation, together withfurther objects and advantages thereof, may best be understood byreference to the following description taken in connection with theaccompanying drawing in which:

FIGURE 1 is a cross-sectional view of a light source embodying myinvention;

FIGURE 2 is a cross sectional view of another light source embodying thepresent invention; and,

FIGURE 3 is a diagram illustrating the energy band- 3,302,051 PatentedJan. 31, l96'7 "ice gap variation in a device fabricated in accord withthis invention.

Photons are emitted in a semiconductive body in response to an electron,that possesses an energy near or equal to that of the conduction band,dropping down in energy by an amount substantially equal to the bandgapof the material, to possess an energy near or equal to that of thevalence band. The photon emitted is substantially equal in energy to theenergy given up by the electron, and consequently is substantially equalto the band-gap energy of the material. The latter defines thedifference in energy between the edges of the conduction and valencebands.

Absorption in a semiconductive material is most commonly associated witha photon giving up its energy to an electron which is consequentlyraised from an energy about equal to that of the valence band to anenergy near or equal to that of the conduction band, resulting in anelectron-hole pair. When the energy of the photon is so low that it isincapable of raising the energy of an electron by an amountapproximately equal to the bandgap energy, this form of photonabsorption does not occur and the semiconductive material appearssubstantially transparent to the photon, though, there are secondaryeffects present of lesser consequence associated with structuraldefects, impurities and free carriers.

In accord with my invention I provide a semiconductive crystal body thatis constituted of an alloy semiconductive material wherein theconcentrations of constituents varies, progressing along the body in atleast one direction. An electroluminescent P-N junction is formed in theregion of the semiconductive body possessing the lower band-gap energy.The remainder of the semiconductive body is then formed or shaped toprovide an efficient structure for extracting the radiation, that iseither coherent or incoherent, from the semiconductive body. Thisportion, by virtue of its greater band-gap energy, is substantiallytransparent to photons originating at the P-N junction.

In general, a semiconductive crystal suitable for use in accord with thepresent invention is, conveniently, fabricated from two mutually solublesemiconductive materials that are mixed, melted, and recrystallized byany of a plurality of known techniques including seed crystal withdrawaland zone refining. The semiconductive constituents are selected toexhibit ditferent band-gaps. Normally, the portion of the semiconductivealloy having the greater concentration of the lower band-gap materialprovides the optimum region for forming the PN junction. The variationin constituent concentrations advantageously progresses slowly, andpreferably uniformly, throughout the body by varying a parameter, astemperature, during the recrystallization phase of the process. Thereason that gradual changes are preferred is to ensure minimumdiscontinuities in the body that can provide reflections and otherundesirable optical and/or solid state effects. The vapor transportcrystal growing technique has proved particularly efiicient in producingcrystals suitable for use in accord with my invention. The constituenthaving the greater dissociation pressure (generally the constituent withthe lower melting temperature) is initially grown in a concentrationthat is less than its presence in the source by a percentage thatincreases as the temperature with which the transport is carried outincreases.

Preferred practice of the present invention is with an alloysemiconductive body constituted essentially of two mutually solubleconstituents selected from the Group III-V and II-VI compounds (of thePeriodic Chart of the elements). Particularly desirable combinations ofthe Group III-V compounds includes (GaAs-GaP), (InAs- GaAs), (InAs-InP)and (GaSb-InSb); while correspondingly desirable combinations of theGroup II-VI compounds include (ZnTe-CdTe), (ZnSe-ZnS), (ZnSe-ZnTe),(CdS-ZnS), and (CdS-CdSe). Alloys constituted of one or more elementalsemiconductive materials, as silicon and germanium, are used to equaladvantage in applications wherein the band-gap of the material issubstantially greater than the energy of the photons being transmitted.Indeed, practice of the invention is not limited to an alloy constitutedof semiconductors but equally includes combinations of metals, asantimony and bismuth, that are mutually soluble and combine to provide asemiconductive material. It is only necessary that a semiconductive bodybe provided having portions thereof of differing band-gap energies,andpreferably, having a gradual, uniform variation in energy differencebetween the valence and conduction band edges throughout the bodyprogressing in a given direction.

Particularly suitable constituents for the semiconductive body used inthe practice of this invention are gallium arsenide and galliumphosphide. The reasons for this selection include the highly desirableelectroluminescence properties of PN junctions formed in alloys of theseconstituents and the complete mutual solubility that enables all alloyconcentrations of these materials to be formed readily by techniqueswell-known in the art including the various controlled directionalcrystallization processes and vapor transport techniques. The lattertechnique has been discovered to offer particular advantages and hasbeen conducted as set forth below.

A container fabricated of inert refractory material, as fused quartz,was filled with a mixture of gallium arsenide and gallium phosphide. Thecontainer was suspended over a gallium arsenide substrate in a sealedenclosure filled with hydrogen chloride gas. The temperature of thegallium arsenide and gallium phosphide was raised to a temperature of850 C. by radiant heating and the gallium arsenide substrate wasmaintained at a lower temperature of 800 C. The gases that filled thesealed container included gallium chloride, arsenic, phosphorus andhydrogen. Under the aforementioned conditions the crystal growth rate onthe substrate was discovered to be about of a millimeter per hour. Thegallium arsenide substrate was then recovered from the sealed containerafter ten hours and a one millimeter thick wafer was cut from theexposed surface of the substrate. The surface of the wafer that had beenfirst grown on the substrate contained 81 mole percent gallium arsenideand 19 percent gallium phosphide. The last portion of the crystal togrow contained 76 mole percent gallium arsennide and 24 mole percentgallium phosphide. The change in concentration occurred gradually andsubstantially uniformly from one surface to the other. Similarly, theband-gap varied substantially uniformly from a lowest band-gap at thesurface containing the lesser concentration of gallium phosphide to amaximum band-gap at the opposite surface. Thus, in the preceding way acrystal was produced particularly suited for use in accord with myinvention.

FIGURES 1 and 2 illustrate light sources in accord with the presentinvention that feature a portion of the semiconductive crystal formed toprovide optical means for extracting radiation originating in a P-Njunction. FIGURE 1 illustrates a reflector structure and FIGURE 2 showsa lens structure.

The light source shown in FIGURE 1 features a semiconductive material ofvariable band-gap having surface 1 ground in the general form of aparaboloid. The opposing fiat surface 2 surrounds a centrally disposedmesa 3 that is advantageously formed by masking and etching. A junction4 is formed in the mesa by any of a plurality of well-known techniques,including diffusion of an acceptor impurity, as zinc in the case of(GaAs-GaP). Suitable electrical contacts 5 and 6 provide the requiredelectrical excitation of junction 4 from electrical source 7 that isrequired to provide generation of photons at 4 junction 4. Radiation 8from junction 4 is reflected by paraboloid curved surface 1 out throughflat surface 2. Surface 1 can include a reflecting coating, and surface2 can include an anti-reflecting coating.

In accord with my invention the band-gap of the semiconductive materialin the vicinity of junction 4 is less than the band-gap of the materialfrom which reflector 9 is constituted. Thus, absorption of radiation 8is minimized in solid reflector 9.

In the event that the light source of FIGURE 1 is fabricated from agallium arsenide-gallium phosphide crystal of the type previouslydescribed, the semiconductive material in the vicinity of mesa 3 isconstituted of about 81 mole percent gallium arsenide, remainderessentially gallium phosphide, and the semiconductive material in thevicinity of the point of symmetry on curved surface 1 is constituted ofabout 76 mole percent gallium arsenide, remainder essentially galliumphosphide. Other concentrations are equally acceptable so long as theminimum gallium arsenide concentration in the vicinity of the junctionis in excess of about 50 mole percent. The precise composition at thejunction, for example, determines the wavelength of radiation. Highestenergy per photon is achieved with approximately a 50/50 composition inthe solid solution under consideration. With these materials, radiation8 incident upon surface 1 with an angle of incidence greater than 16 istotally reflected. Improved directional efficiency is achieved byproviding a reflector on the outer surface of curved surface 1 whereradiation impinges at lesser angles.

In general less absorption of radiation occurs in N-type semiconductivematerials, consequently it is preferred that the optical extractionstructures formed in accord with this invention be of N-typeconductivity material. Also, it is preferable that the impurityconcentration be as small as is consistent with providing an acceptablejunction, in order to reduce impurity absorption, even though thisphenomenon is a secondary consideration.

While the light source of FIGURE 1 that employs a reflector structureoffers optimum luminous efliciency and is therefore a preferredstructure for use in the practice of my invention; my invention can alsobe used to great advantage with a structure as shown in FIGURE 2, thatfeatures a lens. More particularly, the optical structure of FIGURE 2 isoftentimes referred to as a Weierstrass sphere. As shown, a P-N junctionis formed in a semiconductive crystal having a substantially sphericalsurface 11. Suitable electric contacts 12 and 13 are connected to the Pand N portions, respectively, to provide electrical excitation ofjunction 10 from source 14. Radiation 15 is transmitted through theN-type conductivity material, impinges upon spherical surface 11 and istransmitted to an outer medium. Junction 10 is positioned reactive tospherical surface 11 so that all radiation impinges on surface 11 at anangle of incidence less than that at which total reflectivity occurs.

In accord with the present invention the semi-conductive materialpossesses a minimum band-gap in the vicinity of junction 10 and theband-gap is increased, preferably gradually, progressing through thebody toward the left as viewed in FIGURE 2. The energy band-gapvariation across the device of FIGURE 1 in a preferred embodiment isillustrated in FIGURE 3. The energy band-gap is minimum in the junctionregion 1' and increases therefrom in the main body of the crystal Thus,radiation originating in junction 10 is transmitted to spherical surface11 with a minimum of absorption, resulting in optimum efliciency for thelight source.

While only certain preferred features of the invention have been shownby way of illustration, many modificatrons and changes will occur tothose skilled in the art. For example, while the present invention isprimarily concerned w1th light sources, it is evident that the graded'band-gap structures of this invention are equally applicable whereradiation from an external source is to be focussed upon a P-N junctionto provide, for example, a photocell. It is, therefore, to be understoodthat the appended claims are intended to cover this and all other suchmodifications and changes as fall within the true spirit and scope of myinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A light source comprising a crystalline semiconductive bodyconstituted of an alloy semiconductive material; a PN junction in saidbody adapted to provide radiation in response to electrical excitation;the constituents of said alloy being present in the vicinity of saidjunction in concentrations that provide a predetermined band-gap energyin the vicinity of said junction; a portion of said body being adaptedto transmit said radiation; said constituents being present in differentconcentrations in said substantially all of said body other than saidjunction to provide a band-gap energy therein which is greater inmagnitude than said predetermined band-gap energy; and, means forproviding said electrical excitation in a forward direction across saidjunction for producing radiation which is efficiently transmittedthrough said body.

2. The light source of claim 1 wherein the bandgap of said materialincreases gradually and substantially uniformly progressing in adirection perpendicular to and away from said junction.

3. The light source of claim 2 wherein said alloy semiconductivematerial is constituted essentially of a solid solution of galliumarsenide and gallium phosphide.

4. The light source of claim 3 wherein the composition of saidsemiconductive material in the vicinity of said junction includes morethan 50 mole percent gallium arsenide, remainder consisting essentiallyof gallium phosphide; and, the concentration of gallium arsenidedecreases and the concentration. of gallium phosphide increasesprogressing away from said junction,

5. A light source comprising: a semiconductive body constituted of atleast two mutually soluble constituents selected from the groupconsisting of the lll-V and II-VI compounds; said constituents beingpresent in said body in varying concentrations along at least onepredetermined direction to provide a varying band-gap in said body alongsaid direction; and, means for generating light radiation within saidbody in the region of lowest band p- References Cited by the ExaminerUNITED STATES PATENTS 3,l32,057 5/1964 Greenberg l48-33.4

OTHER REFERENCES Applied Physics Letters, December 1, 1962, volume 1,Number 4, pages 82 and 83.

Holonyak et al.: Coherent (Visible) Light Emission From Ga (As Pjunctions.

JAMES W. LAWRENCE, Primary Examiner.

R. JUDD, Assistant Examiner.

1. A LIGHT SOURCE COMPRISING A CRYSTALLINE SEMICONDUCTIVE BODYCONSITUTED OF AN ALOY SEMICONDUCTIVE MATERIAL; A P-N JUNCTION IN SAIDBODY ADAPTED TO PROVIDE RADATION IN RESPONSE TO ELECTRICAL EXCIATION;THE CONSTITUENTS OF SAID ALLOY BEING PRESENT IN THE VICINITY OF SAIDJUNCTION IN CONCENTRATIONS THAT PROVIDE A PREDETERMINED BAND-GAP ENERGYIN THE VICINITY OF SAID JUNCTION; A PORTION OF SAID BODY BEING ADAPTEDTO TRANSMIT SAID RADIATION: SAID CONSTITUENTS BEING PRESENT IN DIFFERENTCONCENTRATIONS IN SAID SUBSTANTIALLY ALL OF SAID BODY OTHER THAN SAIDJUNCTION TO PROVIDE A BAND-GAP ENERGY THERIN WHICH IS GREATER INMAGNITUDE THAN SAID PREDETERMINED BAND-GAP ENERGY; AND, MEANS FORPROVIDING SAID ELEC-F